rework of DataSource loading.

The DataSources that are loaded are now controlled entirely via
configuration file of 'datasource_list', like:
   datasource_list: [ "NoCloud", "OVF", "Ec2" ]

Each item in that list is a "DataSourceCollection".  for each item
in the list, cloudinit will attempt to load:
  cloudinit.DataSource<item>
and, failing that,
  DataSource<item>

The module is required to have a method named 'get_datasource_list'
in it that takes a single list of "dependencies" and returns
a list of python classes inside the collection that can run needing
only those dependencies.

The dependencies are defines in DataSource.py. Currently:
  DEP_FILESYSTEM = "FILESYSTEM"
  DEP_NETWORK = "NETWORK"

When 'get_datasource_list' is called for the DataSourceOVF module with
[DEP_FILESYSTEM], then DataSourceOVF returns a single item list with a
reference to the 'DataSourceOVF' class.

When 'get_datasource_list' is called for the DataSourceOVF module with
[DEP_FILESYSTEM, DEP_NETWORK], it will return a single item list
with a reference to 'DataSourceOVFNet'.

cloudinit will then instanciate the class and call its 'get_data' method.
if the get_data method returns 'True', then it selects this class as the
selected Datasource.

1 files changed, 4 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index aa17a1a6..18934465 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -31,3 +31,7 @@
    via the config file, or user data config file
  - add support for posting data about the instance to a url (phone_home)
  - add minimal OVF transport (iso) support
+ - make DataSources that are attempted dynamic and configurable from
+   system config. changen "cloud_type: auto" as configuration for this
+   to 'datasource_list: [ "Ec2" ]'.  Each of the items in that list
+   must be modules that can be loaded by "DataSource<item>"